This invention is directed to a system for synchronizing video displays. Cathode ray tube displays of the type which develop images under the control of a character generator are illustrative of the type of displays which may be advantageously synchronized by the present system.
Conventional video displays of the type considered herein form part of a television system in which a CRT (Cathode Ray Tube) is raster scanned in synchronism with horizontal rate and vertical rate sync pulses. Part of the CRT's image is developed by conventional video circuitry while another part thereof may be developed by a character generator. For example, a small part of the CRT's image area (the character image area) may display the time, a channel number, or the like under the control of the character generator. The remainder of the image area will usually display a conventional television image.
In order to properly locate characters or other data in the character image area, a conventional character generator is synchronized by vertical rate and horizontal rate sync pulses. As shown in FIG. 1, for example, a character generator 10, operating under the control of a microprocessor 12, sends data to a CRT 13 for displaying the data in a designated character image area 14. Synchronization of the character generator 10 is provided by horizontal rate pulses 15 and vertical rate pulses 16. These pulses, developed in the television receiver (not shown), typically have an amplitude of 50 to 60 volts. Hence, they are applied to buffers 18 and 20 to limit their voltage excursion to predetermined amplitudes, and then applied as sync inputs to the character generator 10.
Although this type of synchronization of the character generator provides adequate performance characteristics, it has drawbacks in terms of expense. The use of buffers 18 and 20 obviously increases the cost of the system. In addition, since the character generator is normally fabricated as part of an integrated circuit chip, two input pins on the chip must be dedicated to receiving the two sync inputs. Including these two pins merely for synchronization purposes also adds to the expense of the system.
Accordingly, it is a general object of the invention to provide an improved system for synchronizing a video display.
It is a more specific object of the invention to provide a less expensive system for synchronizing the display of data on a raster scanned CRT.